1. Field of the Invention
This invention relates to distance relays used to detect the presence of faults in electric power transmission systems and to generate trip signals for circuit breakers in response to faults within protection zones of the relay. In particular, the present invention is directed to a distance measuring relay of the type having loss-of-potential protection which prevents false tripping of the circuit breakers in response to failure of the voltage signal used by the relay.
2. Background Information
The distance relay measures the ohmic value from the relay location to a fault point in an electric power transmission line by using the voltage/current information at the relay location. The current for these measurements is provided by current transformers connected to the transmission line, while the voltage information is obtained from potential transformers. The relay determines the apparent impedance, Z.sub.app, by dividing the voltage by the current. The farther the fault is from the relay the greater will be the apparent impedance. The relay is preset to "reach", that is to generate a trip signal for faults, out to a balance-point Z.sub.c. During normal service conditions, the voltage is high and the current is low. Whenever the apparent impedance, Z.sub.app, is higher than the distance relay setting Z.sub.c, the relay will be in a restraint mode. During a fault, the power system voltage collapses, and the fault current is directly related to the fault distance. The fault current is usually higher than the load current, thus Z.sub.app is less than Zc for a fault within the protection zones of the relay. Under these conditions, the relay will operate and generate a trip signal for the circuit breaker which will interrupt current to the fault.
The voltages used by the distance relay are provided by potential transformers. In practice, fuses are provided in the voltage circuit and situated on the line side of the relay for protecting the voltage transforming devices. One or more of the fuses may blow out for some reason and cause a loss-of-potential condition to the relay voltagate circuit. Under these conditions, the relay receives a zero or unbalanced voltage which is similar to a severe system fault condition and the relay responds with an undesirable trip signal.
Various approaches have been devised for dealing with this loss-of-potential condition, such as voltage comparison, and current supervision. The disadvantages of the solutions to date are either complication of the relay or difficulty in setting parameters, and not being able to detect a three-phase fuse failed condition.
In the prior art voltage comparison approach, the voltages are compared with a reference voltage to generate a signal which blocks the trip if the voltages used for determining impedance do not equal the reference voltage. The disadvantages of this approach are that it requires a separate voltage source/device for the reference voltage and the failure of the reference voltage fuse will also disable the distance relay.
Another voltage comparison approach utilizes a voltage relay which senses a voltage difference between the two ends of the fuse. The voltage relay operates when the fuse blows to block the trip signal. The disadvantages of this approach are that it requires an additional voltage relay, and a racing problem can occur between the operation of the voltage relay and the operation of the distance relay when a fuse blows. That is, a false trip may be initiated before the voltage relay can block the trip.
Another solution to the loss-of-potential condition in a distant relay utilizes an over current relay which operates when current is higher than its preset level to unblock the trip signal. The signal from the overcurrent relay is anded with the trip signal generated from Z.sub.app. Thus, in a loss-of-potential situation, the current will be below the setting of the overcurrent relay and the false trip signal is blocked. The disadvantages of this approach are that it requires an over current relay and the over current relay has to be set higher than the maximum load current but lower than the minimum fault current. This setting criteria can be difficult in some applications.
Yet another approach to the problem utilizes sequence component quantities. More specifically, the zero sequence voltages and currents are used. The zero sequence voltage and negated zero sequence current are anded and applied as a negated input to an AND gate together with the TRIP signal generated as a function of the apparent impedance. Under normal operating conditions, both the zero sequence current and zero sequence voltage are zero and the trip signal is unblocked. For a single phase to ground fault or a phase to phase to ground fault, both the zero sequence voltage and the zero sequence current have nominal values, and again, the trip signal is unblocked. For two-phase or three-phase faults both the zero sequence current and voltage are zero and again the trip signal will be generated. For a blown fuse in one or two phases of the potential measuring device the zero sequence voltage will have a nominal value but the zero sequence current will be zero and, in this instance, the trip signal will be blocked. However, with all three phases blown, the zero sequence voltage is also zero and a false trip signal can be generated.
Accordingly, there is a need for a protective relay which reliably prevents tripping of a circuit breaker in a power transmission line in response to a loss-of-potential condition.
In addition, there is a need for such a protective relay which does not overblock and prevent a bona fide trip.